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WASHINGTON - NASA's Spitzer Space Telescope has captured for the first time enough light from planets outside our solar system, known as exoplanets, to identify signatures of molecules in their atmospheres. The landmark achievement is a significant step toward being able to detect life on rocky exoplanets and comes years before astronomers had anticipated.

"This is an amazing surprise," said Spitzer project scientist Michael Werner of NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif. "We had no idea when we designed Spitzer that it would make such a dramatic step in characterizing exoplanets."

Spitzer, a space-based infrared telescope, obtained the detailed data, called spectra, for two different gas exoplanets: HD 189733b is 370 trillion miles away in the constellation Vulpecula, and HD 209458b is 904 trillion miles away in the constellation Pegasus.

Just as a prism disperses sunlight into a rainbow, Spitzer uses an instrument called a spectrograph to reveal a spectrum by splitting light from an object into different wavelengths. The process uncovers "fingerprints" of chemicals making up the object. The exoplanets Spitzer observed are known as "hot Jupiters" because they are gaseous like Jupiter but orbit much closer to their stars.

The data indicate the two planets are drier and cloudier than predicted. Theorists thought hot Jupiters would have lots of water in their atmospheres, but were surprised when none was found around HD 209458b or HD 189733b. In addition, one of the planets, HD 209458b, showed hints of tiny sand grains, called silicates, in its atmosphere. This could mean the water is present in the planet's atmosphere but hidden under high, dusty clouds unlike anything seen around planets in our own solar system.

"The theorists' heads were spinning when they saw the data," said Jeremy Richardson of NASA's Goddard Space Flight Center, Greenbelt, Md.

"It is virtually impossible for water, in the form of vapor, to be absent from the planet, so it must be hidden, probably by the dusty cloud layer we detected in our spectrum," he said. Richardson is lead author of a paper appearing in the Feb. 22 issue of Nature that describes a spectrum for HD 209458b.

A team led by Carl Grillmair of NASA's Spitzer Science Center at the California Institute of Technology in Pasadena, Calif., captured the spectrum of HD 189733b. A team led by Mark R. Swain of JPL focused on the same planet in the Richardson study and came up with similar results. Grillmair's results will be published in the Astrophysical Journal Letters. Swain's findings have been submitted to the Astrophysical Journal Letters.

"With these new observations, we are refining the tools that we will one day need to find life elsewhere if it exists," said Swain. "It's sort of like a dress rehearsal."

Spitzer teased out spectra from the feeble light of the two planets through the "secondary eclipse" technique. In this method, the telescope monitors a planet as it transits, or circles behind its star, temporarily disappearing from view.

By measuring the dip in infrared light that occurred when the planets disappeared, Spitzer's spectrograph was able to obtain spectra of the planets alone. The technique will work only in infrared wavelengths, where the planet is brighter than in visible wavelengths and stands out better next to the overwhelming glare of its star.

In previous observations of HD 209458b, NASA's Hubble Space Telescope measured changes in the light from the star, not the planet, as the planet passed in front. Those observations revealed individual elements, such as sodium, oxygen, carbon and hydrogen, which bounce around the very top of the planet.

"When we first set out to make these observations, they were considered high risk because not many people thought they would work," said Grillmair. "But Spitzer has turned out to be superbly designed and more than up to the task."